1. Field of the Invention
The present invention relates to an optical pickup apparatus for picking up a recorded information, and more specifically to a pickup apparatus having the so called tilt servo system for maintaining an orthogonal relation between an optical axis of the read-out light beam and the surface of the recording disc.
2. Description of Background Information
In the case of optical information playback system, if an angle between the axis of the reading light beam and the surface of the recording disc deviates from a right angle, the so called crosstalk, that is, the leaking of information of adjacent recording tracks will necessarily occur. This angular deviation from a right angle results from several causes, such as the deformation of the recording disc into an umbrella-like form due to the secular change, and the inclination of the axis of rotation of the disc as the result of deformation of some parts of a disc drive unit of the playback system. Since most of these problems arise after the product is put into the market, the above occurence of crosstalk is rather difficult to prevent.
In order to maintain a precise orthogonal relation between the optical axis of the reading light beam and the surface of the recording disc, the so called tilt servo system has been proposed, in which the angular relation is controlled according to the result of an electronical detection of the above mentioned crosstalk. As an example of this "tilt servo" system, the present applicant has already proposed a technique which is disclosed in Japanese Utility Model application, No. 58-063397. However, in the tilt servo control system of the above utility model application, there was a drawback that a malfunction of the system tends to occur when the optical pickup is located at a position of the recording disc where no recording tracks are provided.